SCUBA QUIZ: What is a SPG?

Answer

The SPG is part of your regulator, it is connected to a SCUBA tank with a high pressure hose. SPG stands for Submersible Pressure Gauge and it measures the pressure in your tank, how much air you have got. Another word for this device is manometer.
During your dive you will use the air from your tank and preferable you would like to breath until the end - so it is important to keep an eye on this piece of dive equipment!

It works like the fuel indicator in the car. When your tank is full it indicates the highest volume possible and when you use the fuel, or air when diving, the needle moves down towards zero = empty tank. In cars as well as in diving it is not a good thing to get close to an empty tank - so the 'dangerous' zone is marked red. Stay out of this zone to prevent problems.

There are two types of indicator you are likely to encounter during your dive travels: PSI and BAR. In the USA you will use PSI. Around the red sea and in Asia you will use BAR indicators. The difference is like the difference between the metric and imperial system (Fahrenheit vs Celsius, meters to feet, etc.) A full tank holds about 3000 PSI / 210 BAR, the red zone will be at 700 PSI / 50 BAR.

Most of the times your SPG will be hanging on the left side of your body when you are geared-up. You do not want to drag it over the ocean floor or that it gets stuck somewhere. Attach your SPG with a clip on your BCD or tuck it under the release of your BCD on your belly. Make sure you can easily look at it to check your air.